EMBRYOLOGY OF CRYPTOBRANCHUS 541 



are homologous: the chief divisions of the brain; the cranial 

 nerves; the eye muscles and their innervation; the chondrocra- 

 nium arising from trabeculae and parachordals, and including 

 olfactory, optic and auditory capsules; the visceral arches; and 

 the essential structures of the circulatory system. If we compare 

 onlj' the amphibia with the fishes the range of resemblances 

 becomes still greater; all the important structures are in essential 

 agreement except those concerned with the outer halves of the 

 paired extremities. 



The argument for a diphyletic origin of the pisces and amphibia 

 is based upon (a) the lack of homology in certain elements of 

 the skull, and (b) the difficulty that is experienced in deriving 

 the amphibian limb from the fin of any known fish. But the 

 resemblances in the skull bones become very close when we con- 

 sider fossil forms, and the trend of increasing knowledge is in 

 the direction of more complete homology rather than the reverse. 

 Moreover we find difficulties almost as great in homologizing 

 cranial elements in different fishes; for these structures are plastic 

 and exposed to environmental influences, and with the radical 

 change from an aquatic to a terrestrial habitat we should expect 

 them to be profoundly influenced. The endoskeleton of the 

 paired appendages presents us with a problem of greater difficulty 

 but here, too, we have to deal with structures that we should 

 expect to be greatly modified in connection with the change of 

 habitat. In view of the wide range of resemblances in important 

 structures one is hardly inclined to consider seriously the idea of 

 a diphyletic origin for the fishes and amphibia. 



The amphibians must have descended from some fish having 

 scales with the potentiality of fusing into bony plates. The 

 dermal bones forming the roof of the skull must have been arranged 

 in pairs on each side of a median suture; for this is the condition 

 found in the most primitive known amphibians (e.g., Branchio- 

 saurus) , The ancestral form must be sought in some fish having 

 the endoskeleton of the paired fins widely protruded from the 

 body, and with pectoral and pelvic members similar. Such 

 fins functioned primitively as paddles; with the adoption of a 

 terrestrial method of locomotion, by creeping or crawling on a 



